Ionic dependence of deplasmolysis in the euryhaline diatom Cyclotella cryptica

Abstract

Cells of C. cryptica become plasmolyzed when subjected to a sudden increase in extracellular osmotic pressure (0.3 to 1.01 osmol/kg). Protoplasts regain their original volume if certain ionic species are present in the plasmolyzing solution. KCl is most important since it is the only electrolyte which allows deplasmolysis when supplied alone, and cells do not deplasmolyze if KCl is omitted. Ca2+, Mg2+, and Na+ are also necessary for high rates of deplasmolysis. Neither light nor ouabain have any effect on deplasmolysis rate in contrast with 2,4-dinitrophenol, which acts as a reversible inhibitor. The characteristics of deplasmolysis are consistent with a mechanism based on energy-dependent uptake of KCl, which is also affected by Na+ and divalent cations.Earlier workers in the field of permeability studies added sucrose to diatom suspensions in ion-containing media and observed plasmolysis followed by rapid deplasmolysis. This was interpreted as evidence for unusually high permeability to sucrose. The present work indicates that diatoms are impermeable to sucrose and that deplasmolysis observed by earlier workers was due to active uptake of ions rather than high permeability to sucrose. It is demonstrated that this technique can also be used for comparative studies of osmoregulation.